2 Stroke engine problem, need help please

millard

Registered user
Joined
Oct 27, 2007
Messages
912
Reaction score
0
Location
Southampton
The bike is Honda CRM250 MK1, 1989.
I bought it about a month back and it was running fine so took it for a trail ride on Salisbury Plain and drowned it in a very muddy puddle. On the way home it started to develop symptoms of an air leak as the revs wouldn’t fully die down when shutting the throttle.
It’s fine when cold, very responsive, but after it warms up the revs don’t die down.
If I raise the needle thereby enrichening the mixture the problem lessens but is still aparent, so it's just masking it rather than fixing it.
Long story short - I rebuilt the bottom end with new rod, big end, small end, mains and seals but the problem still persists. I didn't touch the bore or head as I was advised they're fine.
I have blocked off the exhaust and inlet and pressurised the cases to 10psi and slopped soapy water all over. This revealed a small leak on the reed block gaskets which I’ve sorted but the problem is still there.

I removed the airbox so I could see down the throat of the carb and the slide shuts off fully.
I've stripped the carb three times and cleaned with carb cleaner then blown through with an airline, the jets are all clear.
I reset the float height.
The Power valve and oil pump are set to spec.
I then put some Castrol R in the gearbox to see if there was a leak to crank case from it but there was no smell of the R burning.
Then I filled the cases with petrol but there's no leak from the seals.
The carb is standard jetting and all else is very much standard and was running fine before I drowned it, so I'm fairly sure it was something that happened when I filled the engine with muddy water rather than during the rebuild.

I’m now at a loss as to what to do next.
Someone from another site has suggested a mate as a good local mechanic but he seems too busy as I can't raise him so I'd be happy to take it to a recommended mechanic if anyone else knows one in the Southampton area.
Or does anyone have any suggestions? as I'm much better at breaking four strokes than stinkers.
Cheers
Millard
 
You sure it is not the throttle cable binding? Enough free play at idle?
 
You sure it is not the throttle cable binding? Enough free play at idle?

BR
Def not.
I've had the airbox hose off so I could peer down the throat of the carb when running. The slide snaps shut nicely, even when the revs are too high
 
if it has one, just a suggestion back the tick over right off and see if it still over revs? shit inside the where tick over screw fits? ( nah this wouldnt make it rev higher , because it mechanical) forget this one......


A take it you have checked starter jet, and seen if any crap preventing fuel flow , making it weaker?


getting hotter n revs rise makes me think yer dragging in air from manifold possibly...am not too fermilair wi yer moddle , but these are the problems ive come across wi :boobies stokers in the past....:augie


ugg
 
Last edited:
if it has one, just a suggestion back the tick over right off and see if it still over revs? shit inside the where tick over screw fits? ( nah this wouldnt make it rev higher , because it mechanical) forget this one......


A take it you have checked starter jet, and seen if any crap preventing fuel flow , making it weaker?


getting hotter n revs rise makes me think yer dragging in air from manifold possibly...am not too fermilair wi yer moddle , but these are the problems ive come across wi :boobies stokers in the past....:augie


ugg

Cheers Ugg
I checked all that.
I agree about the air leak, just buggered if I can find out where it's coming from though.
 
try the reed block again?
try for leaks when the engine is hot,
clean the carb all you want, it still may be the problem, they look all ok and clear but they sometimes arnt,
suzuki carbs do this a lot, substitute em and all is good
 
try the reed block again?
try for leaks when the engine is hot,
clean the carb all you want, it still may be the problem, they look all ok and clear but they sometimes arnt,
suzuki carbs do this a lot, substitute em and all is good

Thanks Digg

I've pressurised the cases and slopped soapy water all over each and every component/joint/seal.
I initially got a leak show up on the reed block which I sorted and it improved things slightly but didn't cure it. I did this three times and am convinced now that the reed block and inlet stub are good and air tight.
Am trying to scrounge a carb from the CRM250 forum to eliminate the carb.
cheers
millard
 
Cheers Ugg
I checked all that.
I agree about the air leak, just buggered if I can find out where it's coming from though.

might sound daft but try an airline on area where ye recon it might leak , n if revs rise more than its doin nah ,it could give yer an idea were its suckin in the presurised air bein blasted at it :nenau

but not direct in carb ovcourse:D


might be bollox , but could help , digger06 might say daft twat , but if concentrated air forced at a cracked area it just might show where poss leak is?


ugg
 
might sound daft but try an airline on area where ye recon it might leak , n if revs rise more than its doin nah ,it could give yer an idea were its suckin in the presurised air bein blasted at it :nenau

but not direct in carb ovcourse:D


might be bollox , but could help , digger06 might say daft twat , but if concentrated air forced at a cracked area it just might show where poss leak is?


ugg

No it don't sound daft ya daft twat, i'll try it. :D:D:D

Cheers Ugg
 
My thought is you've got an air leak at the carb top.

Back when I used to spanner for a living we'd spray something all around the carb and manifold to locate an airleak on a running engine, but I can't remember what. :blast
WD40? Carb cleaner? Brake cleaner? :nenau
Been a long time...:rob
 
Muddy water scoured the carb slide & body you're leaking air through your carb top work. Carb gets hotter, gap gets bigger, more air finds it's way in, bypassing jets.
 
My thought is you've got an air leak at the carb top.

Back when I used to spanner for a living we'd spray something all around the carb and manifold to locate an airleak on a running engine, but I can't remember what. :blast
WD40? Carb cleaner? Brake cleaner? :nenau
Been a long time...:rob

Starting fluid, ( Ether) revs increase slightly if you spray it around the area & there are leaks. Be careful though....
 


Back
Top Bottom